The HCF repeat is an unusual proteolytic cleavage signal.

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Abstract
The herpes simplex virus VP16-associated protein HCF is a nuclear host-cell factor that exists as a family of polypeptides encoded by a single gene. The mature HCF polypeptides are amino- and carboxy-terminal fragments of a large approximately 300-kD precursor protein that arise through cleavage at one or more centrally located sites. The sites of cleavage are the HCF repeats, highly conserved 26-amino-acid sequences repeated six times in the HCF precursor protein. The HCF repeat alone is sufficient to induce cleavage of a heterologous protein, and cleavage occurs at a defined site--PPCE/THET--within the HCF repeat. Alanine-scan mutagenesis was used to identify a large 18-amino-acid segment of the HCF repeat that is important to induce cleavage of a heterologous protein. Even though HCF is cleaved, the majority of amino- and carboxy-terminal cleavage products remain tightly, albeit noncovalently, associated. Modulation of this noncovalent association may provide a mechanism for regulating HCF activity. For example, the cleaved products of an alternative mRNA splicing variant of HCF do not remain associated.